How do I merge material slots?
Merge materials in 3 easy steps.

This is your materials list:


RED
GREEN
BLUE


If you want to make all that's BLUE to be RED. 
1) Select the BLUE material. 
2) Use the up arrow till the BLUE is under the RED. 
3) Delete the BLUE by using the minus button. 
Done! 
Now all that was BLUE is now RED.  

The way Blender behaves if you delete a material all the material that used that slot will inherit the slot above unless it is the top slot then it will inherit the one below.